In: Advanced Math
Use the formula for the sum of the first n terms of an arithmetic series to find the sum of the first eleven terms of the arithmetic series 2.5, 4, 5.5, … .
Consider the arithmetic sequence,
2.5, 5, 5.5, …
First term of the sequence is a1 = 2.5
Compute common difference as follows:
d = a2 – a1
= 4 – 2.5
= 1.5
Use the formula for nth term of an arithmetic sequence,
an = a1 + (n – 1)d ...... (1)
Substitute a1 = 2.5, d = 1.5 and n = 11 in formula (1) and compute 11th term of the sequence,
a11 = 2.5 + (11 – 1) × 1.5
= 2.5 + 10 × 1.5
= 2.5 + 15
= 17.5
Use the formula for the sum of first n terms of an arithmetic sequence,
Sn = n(a1 + an)/2 ...... (2)
Substitute a1 = 2.5, an = 17.5 and n = 11 in formula (2) and simplify,
S11 = 11(2.5 + 17.5)/2
= 11 × 20/2
= 110